One time paradox was deadly to the Weeping Angels - if a paradox was created at a location where Weeping Angels were feeding, the paradox would poison and kill the Angels. (TV: The Angels Take Manhattan) However, the Weeping Angels could also attempt to create a time paradox itself to use as a food source. (PROSE: Touched by an Angel)

According to the Fifth Doctor, the universe itself began because of an ontological paradox, since the Big Bang would have been triggered by the explosion of fuel from the future space station Terminus. (TV: Terminus)

Using foreknowledge to try and prevent the future would result in a temporal negation paradox. (AUDIO: Cobwebs)

Guerrilla fighters from an alternate timeline where the Daleks ruled the world came back in time to prevent its creation by assassinating Sir Reginald Styles but unknowingly created their timeline with their assassination attempt. Eventually, the Third Doctor and Jo Grant figured out the truth and convinced Shura not to go through with the plot. Instead, he used his dalekanium bomb to destroy a group of attacking Daleks while the delegates for the peace conference were evacuated. This changed the timeline to a better future. (TV: Day of the Daleks)

While attempting to join the Cult of the Heretic, the Master was told that he would have to kill his own past self to prove his dedication to their cause. To this end, the Master attacked his past self at a point when he was attempting to attack the Time Lord complex on Tersurus, but claimed in hindsight that he had chosen to attack his past self because he knew that the younger Master would survive the attack (although the Cult used this opportunity to use the Masters as pawns in their own plans). (AUDIO: The Two Masters)

While not technically creating her past, Melody Pond was named after her mother, Amy Pond's childhood friend, who was really a future regeneration of her, making her named after herself. She also provided the spark that began her parents' relationship, thereby essentially conceiving herself. Later, when shown that her latest incarnation would use the alias "River Song", she adopted the name for herself. (TV: Let's Kill Hitler) Additionally, River never would have been born at all if she hadn't sacrificed herself in the Library (TV: Forest of the Dead) to save the Doctor's life, because if the Doctor had died that day, he never would have met Amy Pond and Rory Williams, so they never would have spent their wedding night on the TARDIS, and they would not have had a child who was part Time Lord.

After being sent back to 1994 by the Weeping Angels, Mark Whitaker, assisted occasionally by the Eleventh Doctor, Amy Pond and Rory Williams ensured that events that occurred between him and his wife Rebecca happened, in the process causing them to happen. The Angels had sent Mark back in time in hopes he'd create a paradox by saving Rebecca from a car accident that they could feed off of, allowing them to grow stronger and keep doing it to other people until they'd done it to the entire human race, trapping them in a loop where people would shape and alter their pasts from the future. As the death of Rebecca had caused Mark to want to save her and led to his actions in the past, saving her would change both the past and the future. Eventually, the Doctor convinced Mark not to save his wife and using a trap set up by a time-travelling Rory, the group was able to effectively destroy the Weeping Angels and prevent the paradox they planned. Aside from saving Rebecca from a Weeping Angel and staying with her as she died, Mark did nothing. The Angel that managed to escape the Doctor and Rory's trap got trapped in the CCTV system and its attempt to change its past by sending Mark into the past was a paradox also as it created the situation it tried to avoid. (PROSE: Touched by an Angel)

In an attempt to stop the Eleventh Doctor from reaching Trenzalore and release the Time Lords, the Kovarian Chapter of the Silence decided to travel back into the Doctor's past and kill him to retroactively prevent him from reaching Trenzalore. Their efforts to kill him by blowing up the TARDIS and sending a "psychopath", River Song, to murder the Doctor only led to the future they were trying to avoid as blowing up the TARDIS created the very Crack the Time Lords were trying to return through and the Doctor never would've made it to Trenzalore without River's help. (TV: The Time of the Doctor)

In some cases, a time traveller had information that came from the act of time travelling, learning it from a future or past version of himself; thus, the information had no real source, known as the bootstrap paradox. (TV: Before the Flood) An example would be the Doctor's method of saving River Song. Knowing that his future self had given her his sonic screwdriver, the Tenth Doctor was able to preserve her; conversely, his future self gave her sonic screwdriver because he remembered his past self saving her. Neither version truly devised the plan; while the future Doctor remembered the plan, the past Doctor didn't think of it until he deduced what the future Doctor had done. In that same meeting, the Doctor learned from River that it was possible to open the TARDIS doors by snapping his fingers, a fact River was aware of because she had seen the Eleventh Doctor do it, who in turn only knew it was possible because the Tenth Doctor had been informed by River. (TV: Forest of the Dead)

Another example would be how Sally Sparrow received instructions on how to fight the Weeping Angels. Sally created a transcript of the Easter egg that the Tenth Doctor recorded for her; she then gave this transcript to the past Tenth Doctor, who then read from it to create the Easter egg, which Sally would later watch. Neither the Doctor nor Sally actually wrote the transcript. (TV: Blink) In another example, a future Doctor left a message for his past self on the planetHeaven (PROSE: Love and War); in yet another, the Fifth Doctor met the Tenth Doctor and saw him cancel out a supernova with a black hole, which meant the Tenth Doctor remembered seeing himself doing it, allowing him to do so. (TV: Time Crash) A prime example of these types of paradoxes occurred when the TARDIS appeared inside itself and started sending people who entered the TARDIS a short time into the past. The most notable example of a paradox in this instance occurred when the Doctor fixed the problem by waiting for himself to enter the TARDIS to tell himself how to solve the problem; to use the Wibbly lever, whereupon the Doctor used the Wibbly lever and entered the TARDIS to tell himself this answer. (TV: Time)

The Twelfth Doctor was called by Madame Karabraxos to rescue the Teller from the Bank of Karabraxos before its destruction. The Doctor set up the heist and erased his own memory of everything after the phone rang to the start of the heist. As a result, he was unaware he set it up. During the heist, he gave Karabraxos his phone number as well as the information that he was a time traveller to offer his help with her greatest regret when she grew old. Thus Madame Karabraxos got the number to call the Doctor from the Doctor himself and called him for help because he told her to after realising that he'd set the whole thing up. (TV: Time Heist)

After genuinely deciding to change, the Missy incarnation of the Master chose to mortally wound her past self to ensure that he would regenerate into her and make the choices she had made. Due to their time lines being out of synch, the Master and thus Missy would not recall their meeting on the Mondasiancolony ship. Also, during her time on the ship, Missy recalled when a woman had shoved her up against a wall in her previous incarnation and made him promise to always keep a spare dematerialisation circuit on him. She realised that she herself did this and performed the action after learning that the Master had burned out the circuit in his TARDIS, ensuring that she herself had the spare he needed on her at that moment. (TV: The Doctor Falls)

In another example, the Dalek Emperor of the timeline in which Daleks underwent the Mutant Phase unintentionally allowed the introduction of waspDNA into the Dalek gene pool which he had travelled back in time to prevent. The Fifth Doctor realised the mistake and convinced the Emperor to destroy the faulty pesticide he had intended to use on the infected Dalek. In the new timeline, the infection was cured and the Mutant Phase never happened. (AUDIO: The Mutant Phase)

Other examples existed of similar corrections of the previous timeline. On other occasions, though, the Fourth Doctor explained that Sutekh, who was even more powerful than the Time Lords, could destroy Sarah Jane Smith's future easily. He took her to an alternative 1980 where this had happened. (TV: Pyramids of Mars) The Ninth Doctor said the same about the Gelth, though by this time the Time Lords had already disappeared in the Last Great Time War. (TV: The Unquiet Dead)

Some races were only capable of changing the past or future because they existed 'outside' of time, and were therefore not subject to its laws. Examples of this included the Time Lords and the Arboretans, a race of plant-like aliens who travelled back to their birth at the moment of their deaths and could live their lives over and over, correcting past mistakes. Although they were hunted to extinction by Dr. Koel Paddox in his attempt to copy their 'power' and change his past, the Arboreteans refused to contact past generations to make themselves a warlike people that could have opposed his plans as they did not wish to erase their peaceful society. Although Paddox succeeded in sending his mind into his past self, since he was a human who lived 'inside' Time, he couldn't change or influence his life, only watch his past self proceed along the same path as it had before. (PROSE: Festival of Death)

Using the Time Scoop, Borusa was able to remove the first four incarnations of the Doctor from their proper points in the Doctor's timeline and transport them to the Death Zone. Each incarnation of the Doctor was able to continue to exist even with the earlier ones removed, however, the Fifth Doctor demonstrated considerable ill effects from each abduction, claiming to be "whittled away piece by piece", and retreated to the safety of the TARDIS. He also claimed that his timeline had been destabilised and that his existence was threatened by the Fourth Doctor being trapped in a time eddy, even briefly fading from existence before contact with the First Doctor strengthened him. (TV: The Five Doctors)

Making contact with the Cult of the Heretic, the Master agreed to help them in their plan to regenerate the universe if they would permit him to join them in the Anomaly cage. The Cult claimed that they would only allow the Master to join them if he killed his past self, but both sides betrayed the other; the Master claimed that he attacked his past self in a manner that he knew his past self would survive, but then the Cult interfered, attacking the older Master and transferring the past Master into his future self, simultaneously trapping the older Master in his past self, telling the younger Master that the other Master was just another Time Lord. The Cult's intention was to allow the young Master to depart in the body of his own future self and then kill the older Master in his past self, with the plan being that the paradox of the younger Master in his own future self after the death of his past body would unmake the universe, allowing the Cult to recreate it. The scale of the paradox was limited as the future-in-past Master was able to escape before the Cult could kill him, but the two Masters travelling in each other's bodies still caused serious damage to reality. The timeline was damaged beyond repair by the time the Seventh Doctor was able to reveal the truth to both Masters, but the three were able to reach the Anomaly Cage before they were destroyed along with the universe, allowing the Doctor to reset reality back to the way it had been before the Cult's actions destroyed reality. The only consequence of these events was that the future Master essentially created his crippled past incarnation. (AUDIO: The Two Masters)

When Rose Tyler saved her father, Pete, from his death in 1987, several events occurred at once. Earlier versions of the Ninth Doctor and Rose disappeared and the inside of the Doctor's TARDIS was thrown out of normal space-time, rendering it an empty, police box-shaped shell. There were also anachronisms created. The car which should have killed Pete Tyler kept on disappearing, reappearing and following him to repeat the accident. Reapers appeared to eat up people and landmarks in the vicinity and eventually the world. Pete chose to die, restoring the timeline. (TV: Father's Day)

During a visit to Apalapucia, Amy Pond became stuck in a separate timestream in the Two Streams Facility that allowed people to experience the rest of their life in a short amount of time due to the Chen-7 virus. The Eleventh Doctor and Rory Williams worked together to rescue Amy, using the TARDIS to lock onto her timestream and enter it, only to find they arrived 36 years into the future where they meet a much older Amy from an alternate timeline where they'd failed to rescue her. Communicating with her younger self through a mirror, the two Amy's realised that this was because the future Amy refused to help rescue her younger self out of a selfish desire to leave herself. By reminding herself of her love for Rory, young Amy was able to convince her future self to help. Following instructions from the Doctor, Rory was able to bring Amy into her own future and they returned to the TARDIS where the Doctor locked the future Amy out as the TARDIS couldn't sustain the paradox of two Amy's in the same place. Future Amy was killed by the Handbots, but the future was negated by the rescue of her present day self. (TV: The Girl Who Waited)

After being sent to 1938 Manhattan by the Weeping Angels, Rory Williams encountered himself as an old man in the Winter Quay and witnessed himself die. This was caused by the Angels throwing him even further back in time to feed off of his temporal energy. The group realised that if they kept Rory from being sent back in time again and this from happening, it would create a paradox and poison the temporal energy the Angels fed off of, killing them. Rory created this paradox by jumping off the roof of the Quay with his wife Amy Pond. This erased the Quay from existence and killed all but one of the Weeping Angels, which was left very weak. It also restored Amy and Rory to life. However, the Angel sent Rory back to an unknown time, and the Doctor told Amy that creating another paradox wouldn't work, as it would "rip New York apart". (TV: The Angels Take Manhattan)

The Eleventh Doctor, in an effort to change Kazran Sardick into a better man, took his twelve-year old self into his future to see himself as an old man and learn what he would become. The two Kazrans were able to touch without any negative effects and seeing his own future caused Kazran to change what he would become after returning to his own time. (TV: A Christmas Carol)

When Mark Whitaker- trapped in the past after a Weeping Angel sent him back to 1994 from 2011- was visited by his past self in 2001, the paradox when he grabbed his younger self's wrist triggered a fire that burned down the elder Mark's apartment. Shortly after this, the Eleventh Doctor, Amy Pond and Rory Williams were visited by a future version of Rory who had been sent back from two years in the future by another Weeping Angel; the Doctor protected the two Rorys from causing a similar explosion with the sonic screwdriver and then travelled forward to the time that the future Rory had come from, allowing him to send the younger Rory back and preserve history. (PROSE: Touched by an Angel)

The outcomes of time paradoxes in the Doctor Who universe have varied according to the needs of the story. The Virgin New Adventures stated that the Time Lords of Gallifrey evolved before any other sentient being in the universe and lived in the distant past (relative to the timeline). Therefore, they could visit any number of possible futures though they would not affect their own pasts out of fear for undoing that future. (The New Adventures novel Lungbarrow by Marc Platt changed this when it explained that the First Doctor had visited Gallifrey's Dark Times in his personal past.) Fans continue to debate, discuss and theorise as to the nature of time paradox in the Whoniverse.